1. Field of the Invention
The present invention relates to an apparatus suited for dry surface treatment, such as a deposition apparatus for evenly forming a deposited film or a blast treating apparatus for surface treatment, to the both surfaces of a work piece of a plate or bow formed rare earth metal-based permanent magnet or the like.
2. Description of the Related Art
A rare earth metal-based permanent magnet such as an R—Fe—B based permanent magnet, of which an Nd—Fe—B based permanent magnet is representative, is used at present in a variety of fields, because it has a high magnetic characteristic.
However, the rare earth metal-based permanent magnet contains metal species (particularly, R) liable to be corroded by oxidation in the atmosphere. Therefore, when the rare earth metal-based permanent magnet is used without being subjected to a surface treatment, the corrosion of the magnet is advanced from its surface due to the influence of a small amount of acid, alkali and/or water to produce rust, thereby bringing about the deterioration and dispersion of the magnetic characteristic. Further, when the magnet having the rust produced therein is incorporated into a device such as a magnetic circuit, there is a possibility that the rust is scattered to pollute surrounding parts or components.
With the foregoing in view, it is a conventional practice to form a deposited film of aluminum or the like on the surface of a rare earth metal-based permanent magnet for the purpose of providing an excellent corrosion resistance to the rare earth metal-based permanent magnet.
Examples of conventionally known apparatus used for forming a deposited film of aluminum or the like on the surface of a rare earth metal-based permanent magnet, include an apparatus described in U.S. Pat. No. 4,116,161 and an apparatus described in Graham Legge “Ion Vapor Deposited Coatings for Improved Corrosion Protection” Reprinted from Industrial Heating, September, 135-140, 1994. FIG. 13 is a diagrammatic front view (a partially perspective view) of the inside of a vacuum-treating chamber 301 connected to an evacuating system (not shown) in one example of such apparatus. Two cylindrical barrels 305, for example, formed of a mesh net of a stainless steel are disposed side-by-side in an upper area in the chamber for rotation about a horizontal rotational axis 306. A plurality of boats 302, which are evaporating sections for evaporating aluminum as a depositing material, are disposed on a boat support base 304 risen on a support table 303 in a lower area in the chamber.
With this apparatus, a plurality of plate-formed rare earth metal-based permanent magnets 340 as work pieces for example are placed into each of the cylindrical barrels 305, and aluminum is evaporated from the boats 302 heated to a predetermined temperature by a heating means (not shown), while rotating the cylindrical barrels about the rotational axis 306, as shown by an arrow in the figure, thereby forming a deposited film of aluminum on the surface of each of the magnets 340 in the cylindrical barrels 305.
The deposited-film forming apparatus shown in FIG. 13 is capable of treating a large amount of the work pieces and excellent in productivity. However, because the deposition of the depositing material to the magnet is only in one direction due to the apparatus structure, film formation is preferentially to the one surface on a side facing the evaporating section. In particular, where treating the magnets in a plate or bow form, there have been cases that large dispersion in film thickness occurs on between the side facing the evaporating section and the opposite side. This phenomenon is conspicuous for the large-sized magnet having a unit weight of 20 g or the greater because the magnet 340 slides along a barrel inner peripheral surface due to rotation of the cylindrical barrel 305 and its one surface faces the evaporating section at all times as shown in FIG. 14.
Meanwhile, conventionally, blast treating apparatuses have been used in the surface treatment of rare earth metal-based permanent magnets, i.e. in the removal of an oxidation layer formed on the surface, surface cleaning, shot peening for finishing the surface treated film and so on. There are various classifications for the blast treating apparatuses. For example, the tumbler-type apparatus has an injection nozzle such that a plurality of magnets are inserted in a drum of the apparatus so that a blast material is injected to the magnet through an opening of the drum while stirring the magnets by rotating the drum (Japanese Patent Laid-open No. 347941/1999). In such an apparatus, however, the blast material to the magnets is injected only through the drum opening. Accordingly, where treating the magnets in a plate or bow form in particular, there is a case that a large dispersion occurs in the degree of treatment between the respective surfaces similarly to that in the foregoing deposition apparatus.